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Cloud Watching - convection cycles - 10.03.04


Cloud formations can reveal deep secrets of coming weather patterns to observers. Learn the basic structure of the convection cycle and how it serves to build clouds in this article.

The most fundamental force pattern in cloud formation is the convection cycle. The first image shows this as a governing dynamic in all clouds. A convection cycle has two components an expanding and lifting current of warm air moving in the center and many contracting and falling currents of cool air descending from aloft along the outside of the cloud formation.


Fig.1


Fig.1

It is a basic principle in physics that warm air rises. As it rises it carries water vapor upwards on the buoyancy of the currents of warmth. The water vapor is an invisible gas composed of the elements of hydrogen and oxygen in the same ratio that is present in water but the water is in the form of a gas. In the formation of clouds, water is warmed by the Sun and produces the gaseous vapor. The gas expands and rises off of the earth. As it expands it begins to cool off in the evaporation process. To experience this in miniature, take a wet towel on a hot day and swing it around. As it moves through the air and the water in it evaporates and turns to a gas, the towel becomes cool. In the formation of clouds, for every foot of rise of warm water vapor, the temperature drops a specific amount. This means that air that has been warmed and starts to rise is simultaneously cooling as it rises. This drop in temperature keeps the forces in the atmosphere in balance.

For every drop in temperature of the expanding current of air, the water vapor in the form of a warm gas begins to precipitate out in the form of actual microscopic water droplets. The lifting force of the column of warmth cannot support the water in the cooler atmospheric condition so the water vapor precipitates out as actual water droplets. The two processes of evaporation due to the expansive warmth; and cooling and condensing of vapor into droplets as the gas evaporates are happening simultaneously. If this were not so then the warming vapor would lift off of the earth and rise up never to return, depleting the atmosphere of warmth and ending all life. The rising and falling of water in convection cycles is the fundamental phenomenon of the atmosphere.

All clouds have the convection cycle as the basis for their formation. The cloud is composed of droplets that have condensed out of the rising vapor. The form of the cloud reveals the forces that formed it in the many variations of the convection cycle. The following images of clouds will help to understand the fundamentals of cloud formation.


Fig.2


Fig.2

We see a bank of flat clouds against a dark horizon before the sun has risen. These are known as stratus clouds because they are arranged in strata or layers. They show that the air close to the ground before dawn is cool. The moisture is not moving vertically in a convection cell but it sliding along the cool ground. In the lower left corner we see some ground fog still lying in the valley. The little bank of wispy clouds above the stratus bank is showing some of the effects of the moisture rising off of the valley floor it is starting to form the first tendency towards vertical growth that characterizes a convection cycle.


Fig.3


Fig.3

Image 3 is a few minutes later and the stratus layers are beginning to expand upwards. The wispy band is showing the telltale columnar pattern of convection. The little cloudlets in the cloud are rapidly expanding and lengthening upwards to form columns. This pattern of growth will be greatly enhanced once the sun has risen and provides warmth to the clouds for their convection.


Fig.4


Fig.4

Here comes the Sun! The wispy bands are now much more substantial. The stratus bands against the horizon are massing up and forming a bank of clouds. Above the growing wispy band another finer layer of very delicate gauze like clouds is coming into form like curdled milk congealing on a glass slab.


Fig.5


Fig.5

The middle layer of wispy clouds is disappearing as the fog on the valley floor is rising. Both of these phenomena point to increased vertical motion. The curdled milk clouds in the upper part of the picture are now gaining substantiality even as the lower clouds are disappearing. Often, in the atmosphere, a bottom layer's warmth and moisture is sublimated upwards to enable the unfolding of the upper layer's formation, that is, the lower clouds disappear even as the higher clouds are forming out of their sublimated water vapor and warmth.


Fig.6


Fig.6

A few minutes later the upper layer has stabilized and the middle layer once again is growing as the Sun is starting to warm the atmosphere. The middle layer is now being fed water vapor once again from the ground as the action of the Sun becomes more pronounced. The fog in the lower left is spreading out along the creek in the valley feeding the cloud formation as the warming air draws up the moisture that had settled over night.


Fig.7


Fig.7

The Sun has risen. The fog is lifting to the tree- tops. Currents of warmth carry moisture into the middle layer as the upper layer now sublimates. Warmth and cold are working into each other to balance the forces in the atmosphere as the middle layer grows substantially.


Fig.8


Fig.8

The Sun is now fully risen and warming the atmosphere vigorously. The wispy clouds have risen much higher and the middle layer is drawing up the rising moisture from the earth and forming it into dense masses. The stratus form of the earlier sequences is now not so visible but the cumulus or heaped cloud pattern is dominant. The fog has spread above the trees and is feeding this rapid growth in the sky above


Fig.9


Fig.9

Cumulus clouds have massed and are now drifting on the wind. High above, sublimated moisture is forming ice clouds as the fog completely obscures the valley floor as it lifts heavenward to form new clouds.